1. Field of the Invention
The present invention relates to an image control device for a printer and a method of compensating for a light amount drift of a photosensor used in the image control device.
2. Description of the Related Art
A printing machine, such as a printer or a copier, forms an electrostatic latent image by projecting an optical signal corresponding to image information onto a photosensitive medium that is charged with a uniform electrical potential using an exposing apparatus, forms a toner image by developing the electrostatic latent image using a developer, transfers the toner image to a recording medium directly or via an intermediate transfer medium, and fixes the toner image onto the recording medium by compressing and heating the toner image. In this way, the printing machine prints an image on the recording medium.
A color toner image on which yellow (Y), cyan (C), magenta (M), and black (K) toners are overlapped is thereby formed to print a color image. A printing process must be precisely controlled to form a color toner image on which color toners have been accurately overlapped to generate a high-quality image. Detection and adjustment of a color registration error is further needed to precisely control the printing process.
A color registration error is generated due to several factors, such as an error in localization of a plurality of developers that contain a plurality of color toners, an error in the manufacture of lenses used in an exposure apparatus, an error in the driving of a photosensitive medium or an intermediate transfer medium, and the like.
To generate a high-quality image, a concentration of an image must be appropriately adjusted. In other words, if an error is detected when a concentration of input image information is fully reflected in a toner image on an intermediate transfer medium, the error must be compensated for by adjusting the amount of exposed light, a developing bias applied to a developer, and the like.
In general, an image control mark including a color registration mark and an image concentration mark is formed on the intermediate transfer medium. A color registration error and an image concentration error can then be detected by detecting the image control mark using a photosensor. As shown in FIG. 1, a photosensor 3 comprises a light emitting portion 1 and a light receiving portion 2. Light emitted from the light emitting portion 1 is reflected by an image concentration mark (or a color registration mark) 5, which is formed on a transfer belt 4, and incident upon the light receiving portion 2. To accurately detect an image concentration (or a color registration error), the light emitting portion 1 and the light receiving portion 2 must be provided at a precise location during production of each photosensor 3. However, in practice, the location of each of the light emitting portion 1 and the light receiving portion 2 is can vary, or drift. A location of the image concentration mark (or the color registration mark) 5 of the photosensor 3 can also vary, or drift.
When the amount of light emitted from the light emitting portion 1 is constant, the amount of light detected by the light receiving portion 2 must be constant so that the color registration error and the image concentration error can be accurately detected. A drift of the location of either the light emitting portion 1 and the light receiving portion 2 impedes an accurate detection of the color registration error and the image concentration error. Thus, precise image control is difficult.
The installation of a compensation circuit (not shown) in the photosensor 3 may be considered to compensate for a drift of the location of each of the light emitting portion 1 and the light receiving portion 2. However, this solution increases the price of the photosensor 3. Also, even if the compensation circuit is used to compensate for the drift of the location of each of the light emitting portion 1 and the light receiving portion 2, the compensation circuit cannot compensate for a drift of the location of the image concentration mark (or the color registration mark) 5.
Accordingly, a need exists for a system and method for compensating for both a drift of locations of the light emitting portion and the light receiving portion of a photosensor, and a drift of a location of an image control mark with respect to the photosensor.